The present invention relates to a process for the skeleton isomerization of n-alkenes to isoalkenes, i.e., for the conversion of unbranched hydrocarbons into branched, unsaturated hydrocarbons.
The demand for isoalkenes, especially in the C.sub.4 - to C.sub.5 -range, has risen considerably in recent years due to the development of new techniques, a change in the raw material situation, and legal requirements enacted in the fuel sector. Examples of the new developments are the methyl tert-butyl ether synthesis, which is a simple processing technique for the C.sub.4 -cut with the simultaneous production of a valuable fuel component; and processes for the manufacture of methyl methacrylate, starting with isobutene or tert-butanol.
The skeleton isomerization of n-alkenes to isoalkenes is a conventional reaction. The conversion is normally conducted as a heterogeneously catalyzed gas-phase reaction in the temperature range of 300.degree. to 600.degree. C. Catalysts employed are acidic solids, e.g., aluminum oxides acidified by suitable promoters. The aluminum oxide is preferably utilized in the .eta.- or .gamma.-modification.
However, problems are encountered in the rapid, extensive coking of the catalyst which ensues, resulting in very brief periods of operation in between the necessary frequent regenerations. For example, during the isomerization of n-butane to isobutane, carried out on an extremely large scale under practical conditions, the catalyst is even doped with Pd or Pt, and hydrogen is added to the hydrocarbon mixture, so that the olefins produced in secondary reactions can be hydrogenated as quickly as possible again to alkanes. Without such measures, the period of usefulness for the catalyst drops from about 1/2 year to a few days, because the olefins lead to rapid coke formation on the catalyst. For this reason the heretofore known processes for butene isomerization have not as yet been employed on an industrial scale.
For example, DAS [German Published Application] No. 1,518,580 describes a process for butene isomerization wherein the conversion is conducted in the presence of an aluminum oxide which contains 0.5-1.5% by weight of fluorine at 250.degree.-550.degree. C. The short lifetime of the catalyst is a disadvantage in this method. As a consequence of coke deposits, the activity is considerably reduced within only a few hours. Moreover, for regeneration purposes, the catalyst must be burnt off with air.
DOS [German Unexamined Laid-Open Application] No. 2,534,459 describes a process wherein the alkene is brought into contact with a catalyst obtained by reacting an activated aluminum oxide with an ester of silicic acid. The results of ths process are that, here again, periodic regenerations are required at intervals of 10-20 hours.
Furthermore, from DAS No. 1,518,584 it is known to conduct the isomerization in the presence of hydrogen as a diluent gas as a measure for prolonging the useful life of a fluorinated aluminum oxide catalyst. The half-life of the catalyst, defined as the point at which the percentage content of isobutene, based on the total butene in the gaseous product, has dropped to half its equilibrium value at 450.degree. C., is disclosed as 69 hours. When using inert gases, such as H.sub.2 or N.sub.2, however, the separation of the inert compounds from the C.sub.4 -hydrocarbons is expensive since this normally requires the use of refrigeration. Furthermore, additional safety measures are required in the necessary regeneration procedure to avoid the occurrence of explosive gaseous mixtures.